Two Visual Mechanisms of Photosensitivity


Address correspondence and reprint requests to Dr. G. F. A. Harding at Clinical Neurophysiology Unit, Aston University, Birmingham, B4 7ET, U.K.


Summary: Purpose: Photosensitive epilepsy is the most common of the “reflex” epilepsies. Precipitated by television viewing, flickering light, or specific visual patterns, it is the cause of seizures in 10% of young people with epilepsy. Photosensitivity is associated with two types of EEG abnormalities: photoparoxysmal responses (PPRs) and occipital spikes (OSs). It is unclear whether these abnormalities are mediated by different mechanisms, and furthermore, the clinical significance of OS is unknown.

Methods: By using our previously established population of patients with photosensitive epilepsy, all showing EEG abnormalities on intermittent photic stimulation or pattern stimulation, we examined the effects of pattern contrast, spatial and counterphase temporal frequency, and colour on these abnormalities.

Results: PPRs and not OSs show linear contrast dependency and are elicited by stationary stimuli and by non-colour-opponent isoluminant stimuli.

Conclusions: PPRs and OSs are generated independently by the parvocellular and magnocellular visual systems, respectively. The results add support to the hypothesis that only PPRs and not OSs are clinically significant.